Looking at your website, what appeared to happen is something else. Element classes (like your thumbnail-hover-appear class or your portfolio-heading class) are not page-specific. That means that even if you're working on a duplicate of a page, any changes to the CSS will apply to all instances of those classes regardless of what page they're on. HTML-specific changes on the other hand, like adding or removing text, reorganizing content, etc., are specific to that page.
Edit: When you say "experiment on the copy" do you mean just changing the text itself? Or were you changing properties associated with that copy and other elements (styling/size/margin)?